Because there are so many ways to live polyamorously!!
I don't want to co-opt your intro thread, but since you asked...
If there's some kind of flag to wave and a symbol that somehow becomes generally acknowledged to represent polyfolk, there will certainly be ideas and assumptions that go with it and most of them will either be wrong, or blanket generalizations that simply cannot apply to everyone who is poly. I don't want to be lumped into some category people will make up about what they think my life must be because they've seen a symbol and been told it represents some idea of "The Poly Lifestyle."™ Not that I spend much time worrying about how people would judge me, but I'd rather not swim upstream if I can help it. I think a symbol furthers the notion that having multiple relationships is weird and abnormal, because it means we belong to some subculture that is separate from everyone else.
How can a symbol represent a very diverse group anyway? It's no wonder there have been so many designs and none have stuck for good.
As it is, I don't usually use the word polyamory to describe how I want to live because most people have incorrect ideas about it. They think I'm a swinger, that I just want threesomes, that I can't be straight, that I'm not into commitment or long-term relationships, and so on. So, I have to start off talking about exclusivity and then non-monogamy before even mentioning the p-word. I just want to be assessed and related to at face value for who I am, not some idea people might assume about me because I belong to an unconventional "club" with a symbol and a flag. That's about it.